It is known that a lithographic printing plate having an improved receptivity to printing inks can be produced by using a light-sensitive layer made of a light-sensitive material composed of a high-molecular weight compound and a water-insoluble diazo resin. See, for example, Japanese Patent Publication No. 1167/1972, and Japanese Patent Applications (OPI) Nos. 9804/1973, 24404/1972, 38302/1972, 30604/1975, 118802/1975 and 120903/1978 (the symbol OPI as used herein means an unexamined published Japanese patent application).
The water-insoluble diazo resin used in the light-sensitive layer is in most cases an organic acid salt (typically an aromatic sulfonic acid salt) of a condensate of an aromatic diazonium compound and a carbonyl-containing organic compound. However, few light-sensitive layers made of this water-insoluble diazo resin and a high-molecular weight compound are used commercially. The reason is that a presensitized lithographic printing plate prepared by applying this light-sensitive layer to a support having a hydrophilic surface, and particularly an aluminum plate preliminarily treated to render it hydrophilic, deteriorates upon standing, and its developability is decreased to such an extent that the non-image areas are stained during printing. In order to eliminate this defect, several stabilizers have been proposed for incorporation in the light-sensitive layer such as a phosphorous acid, as described in Japanese Patent Application (OPI) No. 151023/1979; oxalic acid as described in U.S. Pat. No. 4,172,729; a halogen-containing organic phospate ester compound as described in Japanese Patent Application (OPI) No. 36207/1975 corresponding to German Patent Application (OLS) No. 2,437,408 a heterocyclic diazonium salt as described in Japanese Patent Application (OPI) No. 143405/1976, and phosphoric acid, sulfuric acid, organic sulfonic acid, polyacrylic acid, polyvinyl phosphonic acid and polyvinyl sulfonic acid, as listed in U.S. Pat. No. 3,679,419 (page 6, line 69--page 7, line 7). These compounds have proved to have the ability of stabilizing the light-sensitive layer described above when they are incorporated therein.
It is known that when a salt-forming organic compound is added as a dye to the light-sensitive layer made of a diazo resin of an organic acid salt and a high-molecular weight compound, the layer is provided with such print-out property that enables an immediate image recognition upon exposure. However, the "print-out property" of the diazo resin of an organic acid salt is weak, and this problem can be solved by using a diazo resin of a halogenated Lewis acid salt as described in U.S. Pat. No. 3,294,533 and Japanese Patent Application (OPI) No. 98613/1979.
However, it has turned out that the stabilizers listed above which would be effective for use with the diazo resin of an organic acid salt are not highly effective for use in light-sensitive layer of good "print-out" property which are made of the diazo resin of a halogenated Lewis acid salt in combination with a high-molecular weight compound.